PROJECT SUMMARY Mutations in the SCN1A voltage-gated sodium channel (VGSC) are responsible for a growing number of disorders, including genetic epilepsy with febrile seizures plus (GEFS+), Dravet syndrome (DS, or severe myoclonic epilepsy of infancy), and familial hemiplegic migraine. To better understand the mechanism by which SCN1A dysfunction leads to epilepsy, we generated transgenic and knock-in mice with the human SCN1A mutation R1648H. These mutants exhibit spontaneous seizures, reduced seizure thresholds, and shortened life spans. Electrophysiological analysis of cortical neurons revealed reduced function in both excitatory and inhibitory neurons, but the biophysical mechanisms were different - negatively shifted voltage dependence of fast inactivation in excitatory neurons versus slowed recovery from inactivation in inhibitory neurons. Our results and data from other groups led to the hypothesis that reduced GABAergic inhibition plays the major role in the pathogenesis of GEFS+ and DS. However, a direct causal link between SCN1A function in interneurons or pyramidal cells and seizure generation has not yet been established. In Aim 1, we will selectively delete Scn1a from either interneurons or pyramidal cells to directly establish the relative contribution of each cell type to seizure generation. In Aim 2, we will test the hypothesis that early-life febrile seizures (FSs), which are a prominent clinical feature of both GEFS+ and DS, have an impact on disease progression. We will also explore the mechanistic basis for the relationship between FSs and disease outcome and possible pharmacological interventions. Of further relevance to our ultimate goal of finding better treatments for epilepsy is our observation that altered function of the Scn8a VGSC can restore normal seizure thresholds and life spans to an Scn1a knockout model of DS. This observation led us to hypothesize that selective targeting of SCN8A may make an effective treatment for DS, which is often refractory to available medications. In Aim 3 we will investigate whether altering Scn8a function can also ameliorate other genetic forms of epilepsy and types of seizures. This study will provide important information on the role of VGSCs in the maintenance of normal neuronal excitability and in the development of epilepsy. These experiments are innovative, clinically relevant, and will stimulate much-needed translational research.